1. Field of the Invention
The present invention relates to thin diamond films. More specifically, the present invention relates to continuous thin diamond films, and methods for making continuous thin diamond films.
2. The Prior Art
Diamond films of a sort have been reported in the literature. However, continuous diamond films having thicknesses substantially less than 1 micron over large areas (i.e. 1 cm.sup.2) appear to have been beyond the reach of the art. The reported methods for producing diamond films have included plasma enhanced chemical vapor deposition (CVD) techniques utilizing RF energy, microwave energy, direct current techniques or hot wire filament methods with or without plasma assist.
Methods for producing diamond films have included the decomposition of a hydrocarbon gas such as methane, in the presence of hydrogen, and the consequent formation of diamond crystals on a substrate material in a CVD reactor. The substrate materials have been first prepared, usually by mechanical polishing using diamond dust, in order to create suitable nucleation sites for the thin diamond film. Alternatively, thin diamond films have been epitaxially grown on existing diamond crystals. Metal substrates, as well as silicon and other semiconductor substrates, have been utilized.
A key uncertainty in the growth of diamond films has heretofore been success or not in nucleating diamond on a variety of substrates. Typically, in order to promote nucleation, the substrate has been abraded with diamond or silicon carbide powder so as to provide a high density of energetically favorable nucleation sites on the substrate surface. There are several drawbacks to this approach. Abrasion damages the substrate surface since the objective of abrasion is to provide locally damaged regions on the substrate to promote diamond film nucleation. Whereas this approach to film nucleation may be acceptable in those applications that are not influenced by the damaged regions on the substrate, in a majority of applications where the quality of the interface between the substrate and the diamond film is important, surface abrasion has not proven viable as an approach to the promotion of diamond film nucleation.
Where diamond films are to be used in conjunction with other semiconductors such as silicon or gallium arsenide for applications as capacitors, passivation layers, and heat transfer elements, the quality of the interface between the semiconductor and the diamond film is critical. Other potential applications where good interface properties are important include the diamond coating of infrared optical materials for protection against abrasion. In such cases the optical properties of the infrared materials may be compromised by the presence of a damaged layer at the interface.
In addition to mechanical damage at the interface, substrate preparation by the use of abrasives such as diamond or silicon carbide is likely to result in a contaminated surface, a feature that is also detrimental to the creation of high quality interfaces. Cleaning the surface following abrasion can reduce the effectiveness of the locally damaged regions by the saturation of dangling bonds by the cleaning agents. Finally, the process of abrasion is difficult to control and can be considered to be more art than science, with consequent unpredictability in application.
As a consequence, approaches for appropriately treating the surface of the substrate to enhance the nucleation of diamond crystals are needed.
The physical and electrical properties of the diamond form of carbon make it a candidate for numerous potentially exciting uses. Among these uses are as semiconductor and insulating films in electronic devices, and radiation windows useful in various scientific applications.
For instance, the study of X-ray spectra from light elements, including carbon, boron, nitrogen, and oxygen, requires a window material which will pass a sufficient number of X-ray photons from a sample to allow collection of meaningful data. Prior to the present invention, such windows have been fabricated from boron nitride, beryllium, and from certain polymer films.
While these materials have proved to be useful, each suffers from one or more drawbacks. For instance, the properties of boron nitride are such that windows larger than about 4 mm in diameter have generally not been practicable. In addition, boron nitride contains appreciable quantities of hydrogen, which contribute to its degradation over time.
Beryllium windows, which are the most commonly used today, apparently cannot be made thinner than about 8.0 microns without appreciable leakage. At thicknesses less vulnerable to such defects, such windows will not allow low energy X-rays from light elements such as carbon, oxygen, nitrogen and boron to pass through them. Consequently, X-rays from these elements cannot usefully be detected using beryllium windows. In addition, beryllium is easily subject to corrosion from exposure to even small amounts of water vapor in the laboratory environment.
Thin windows have been made using polymeric material, but these windows suffer from drawbacks. Polymer films, while not subject to water vapor corrosion, are subject to fatigue fracture after being repeatedly cycled between atmospheric pressure and the pressure used in X-ray analysis equipment.
To our eyes, the properties of diamond suggested to us that it could be an excellent candidate for a window material in the X-ray environment, especially for observation of X-ray spectra from the light elements. A thin diamond film can be expected to be corrosion resistant, and, if it could be properly fabricated, could have enough strength to withstand the stress of repeated pressure cycling encountered in X-ray analysis equipment. A thin diamond film, having a thickness of about 5,000 .ANG. or less, will have an X-ray absorption fine structure which will allow passage of sufficient numbers of low-energy X-ray photons to enable it to be used as a window for X-ray instrumentation.
The prior art has apparently been unable to provide a continuous thin diamond film sufficiently thin (less than about 2 microns thick) to enable it to be employed in this and other applications. For certain applications, such a film must be sufficiently defect free so that it will have very poor permeability, viz., be a low-leakage material, and have sufficient strength to withstand both absolute pressure differentials and repeated pressure cyclings in the X-ray environment. These properties in a thin diamond film would also qualify it as a good candidate for other applications requiring high quality diamond thin films.